1. Field of the Invention
The present invention relates to a cassette for housing a sheet-like workpiece therein.
2. Description of the Related Art
There are known systems for recording radiation image information of a subject such as a human body on a stimulable phosphor (photo-simulated emission phosphor) and either reproducing the recorded radiation image information on a photosensitive medium such as a photographic film or the like, or outputting the recorded radiation image information as a visible image on a CRT or the like.
The stimulable phosphor is a phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to applied stimulating rays such as visible light, emits light in proportion to the stored energy of the radiation. The stimulable phosphor is usually arranged in the form of a sheet for use as a stimulable phosphor sheet.
It has been customary to apply a radiation, e.g., X-rays, to a subject such as a human body or the like and record the radiation image information of the subject directly on a photographic film. A radiation image recording medium such as a stimulable phosphor sheet, a photographic sheet, or the like is normally housed in a cassette loaded in a photographic device, and X-rays are applied to the radiation image recording medium through the cassette. For reading the radiation image information recorded on a stimulable phosphor sheet, for example, the cassette is loaded into an image reading apparatus. Then, stimulating light is applied to the stimulable phosphor sheet, and light emitted from the stimulable phosphor sheet and representing the stored radiation energy is photoelectrically read and converted into an electric signal.
Japanese Laid-Open Patent Publication No. 11-271894 discloses a cassette comprising a casing and a lid that is openably and closably mounted on the casing for removing a stimulable phosphor sheet from the casing and storing the stimulable phosphor sheet back into the casing.
The casing accommodates therein a pair of lock mechanisms respectively disposed with a certain distance from the center of the front face thereof for locking the lid against opening and closing movement. The lid can be opened when it is unlocked by a pair of unlock functions mounted on a front face of the casing.
When a portion of the casing, particularly, an end of the casing, is opened, the casing provides a large open area. Because of the large open area, the casing as it is open is not sufficiently rigid and is not fully shielded against light which tends to enter the casing through the open area.
In addition, inasmuch as the lock mechanisms for locking the lid against opening and closing movement are disposed with a certain distance from the center of the front face of the casing, the lid is not locked against movement substantially centrally in the casing, and such a cassette is likely to suffer a lack of strength when the lid is closed.
Some cassettes have an openable and closable lid which is provided by an entire side panel of the casing, rather than by only a portion of the casing. With such a cassette, since a large space is required to open and close the lid, an image reading apparatus for reading recorded radiation image information from the stimulable phosphor sheet stored in the cassette is liable to large in size due to the large space required to open and close the lid.